This invention is directed to radiography and in particular to imaging assemblies (or xe2x80x9ccassettesxe2x80x9d) and radiographic imaging assemblies that are useful for oncology portal imaging. Thus, this invention is useful in portal radiography.
In conventional medical diagnostic imaging the object is to obtain an image of a patient""s internal anatomy with as little X-radiation exposure as possible. The fastest imaging speeds are realized by mounting a dual-coated radiographic element between a pair of fluorescent intensifying screens for imagewise exposure. About 5% or less of the exposing X-radiation passing through the patient is adsorbed directly by the latent image forming silver halide emulsion layers within the dual-coated radiographic element. Most of the X-radiation that participates in image formation is absorbed by phosphor particles within the fluorescent screens. This stimulates light emission that is more readily absorbed by the silver halide emulsion layers of the radiographic element.
Examples of radiographic element constructions for medical diagnostic purposes are provided by U.S. Pat. No. 4,425,425 (Abbott et al.) and U.S. Pat. No. 4,425,426 (Abbott et al.), U.S. Pat. No. 4,414,310 (Dickerson), U.S. Pat. No. 4,803,150 (Kelly et al.) and U.S. Pat. No. 4,900,652 (Kelly et al.), U.S. Pat. No. 5,252,442 (Tsaur et al.), and Research Disclosure, Vol. 184, August 1979, Item 18431.
Radiation oncology is a field of radiology relating to the treatment of cancers using high energy X-radiation. This treatment is also known as teletherapy, using powerful, high energy X-radiation machines (often linear accelerators) or cobalt (60) units to exposure the cancerous tissues (tumor). The goal of such treatment is to cure the patient by selectively killing the cancer while minimizing damage to surrounding healthy tissues.
Such treatment is commonly carried out using high energy X-radiation, 4 to 25 MVp. The X-radiation beams are very carefully mapped for intensity and energy. The patient is carefully imaged using a conventional diagnostic X-radiation unit, a CT scanner, and/or an MRI scanner to accurately locate the various tissues (healthy and cancerous) in the patient. With full knowledge of the treatment beam and the patient""s anatomy, a dosimetrist determines where and for how long the treatment X-radiation will be directed, and predicts the radiation dose to the patient.
Usually, this causes some healthy tissues to be overexposed. To reduce this effect, the dosimetrist provides one or more custom-designed xe2x80x9cblocksxe2x80x9d or shields of lead around the patient""s body to absorb X-radiation that would impact healthy tissues.
To determine and document that a treatment radiation beam is accurately aimed and is effectively killing the cancerous tissues, two types of imaging are carried out during the course of the treatment. xe2x80x9cPortal radiographyxe2x80x9d is generally the term used to describe such imaging. The first type of portal imaging is known as xe2x80x9clocalizationxe2x80x9d imaging in which the portal radiographic film is briefly exposed to the X-radiation passing through the patient with the lead shields removed and then with the lead shields in place. Exposure without the lead shields provides a faint image of anatomical features that can be used as orientation references near the targeted feature while the exposure with the lead shields superimposes a second image of the port area. This process insures that the lead shields are in the correct location relative to the patient""s healthy tissues. Both exposures are made using a fraction of the total treatment dose, usually 1 to 4 monitor units out of a total dose of 45-150 monitor units. Thus, the patient receives less than 20 RAD""s of radiation.
If the patient and lead shields are accurately positioned relative to each other, the therapy treatment is carried out using a killing dose of X-radiation administered through the port. The patient typically receives from 50 to 300 RAD""s during this treatment. Since any movement of the patient during exposure can reduce treatment effectiveness, it is important to minimize the time required to process the imaged films.
A second, less common form of portal radiography is known as xe2x80x9cverificationxe2x80x9d imaging to verify the location of the cell-killing exposure. The purpose of this imaging is to record enough anatomical information to confirm that the cell-killing exposure was properly aligned with the targeted tissue. The imaging film/cassette assembly is kept in place behind the patient for the full duration of the treatment. Verification films have only a single field (the lead shields are in place) and are generally imaged at intervals during the treatment regime that may last for weeks. Thus, it is important to insure that proper targeted tissue and only that tissue is exposed to the high level radiation because the levels of radiation are borderline lethal.
Portal radiographic imaging film, assembly and methods are described, for example, in U.S. Pat. No. 5,871,892 (Dickerson et al.) in which the same type of radiographic element can be used for both localization and portal imaging.
Some of the earliest portal radiation images were recorded on xe2x80x9cdirectxe2x80x9d radiographic films that were designed for industrial purposes. These films were often placed in cardboard film holders and required special processing techniques. However, such imaging xe2x80x9cassembliesxe2x80x9d had the advantage of being lightweight and easy to carry. To address this problem, Eastman Kodak Company introduced films in the early 1970""s in light-tight envelopes and ready for use in portal and localization imaging. These films could be used without a film holder or cassette and were even more lightweight and convenient to use because they could be processed using conventional techniques and equipment. However, the images obtained at higher energies were poor in quality.
As high energy linear accelerators became more commonly used, it was determined by several researchers in the field that metal plates should be used during portal imaging (see for example, Hammoudah et al., Int.J Radiation Oncol. Biol. Phys. 2, 571-577, 1977 and Droege et al., Medical Physics, 6, pp. 487-493 and pp. 515-518, 1979). Consequently, several years later, several manufacturers introduced imaging assembles (xe2x80x9ccassettesxe2x80x9d) for portal and localization imaging that included either or both front and back metal plates. These imaging assemblies provided improved images at higher treatment energies, but because of the inclusion of the one or more metal plates, they were much heavier and difficult to handle and transport throughout the medical community.
Even with the inclusion of the heavier metal plate, the final image contrast was observed to be too low. Current imaging assemblies commercially available include a 1 mm copper front metal screen, front and back gadolinium oxysulfide fluorescent intensifying screens and a back lead metal screen. These imaging assemblies also include a high contrast fine grain silver halide radiographic film to provide much improved portal images. However, the weight of such imaging assemblies is considerable and creates a problem for users in the medical imaging community.
Since the earliest teaching about the need for metal screens in the imaging assemblies, the thickness of the metal screens has been set at 1 mm or more. It was consistently believed that thick metal screens were required to avoid overexposure especially for portal imaging. Thus, commercial products have consistently included at least 1 mm-thick metal screens in the front of the imaging assembly.
Presently, in portal imaging, the multi-MVp X-radiation in part passes through the patient unabsorbed and is in part absorbed creating a secondary electron emission. The front metal screen is usually relied upon to intercept and absorb the secondary electron emission. This lowers minimum density and significantly enhances image sharpness. Image intensification (raising maximum density and contrast) is achieved by absorbing X-radiation and transmitting to the radiographic film the electrons that are thereby generated.
In addition to the front metal screen, which is always present, a back metal screen can also be employed to provide an additional source of electrons for radiographic film exposure. Conventional portal imaging assemblies can be grouped into two categories. In the first category, the imaging assembly includes one or two metal plates and a photographic silver halide film that is designed for direct exposure to X-rays or electrons. Two such films are commercially available from Eastman Kodak Company as KODAK X-RAY Therapy Localization (XTL) Film and KODAK X-RAY Verification (XV) Film. Such direct X-ray exposure assemblies are illustrated in FIG. 1 (described in more detail below). A commercial imaging assembly of this type is commercially available as KODAK EC-L Cassette from Eastman Kodak Company.
The second portal imaging assembly uses a radiographic photographic silver halide film containing fine grain silver halide emulsions, one or two fluorescent (or phosphor) screens (or intensifying panels) and one or two metal plates. One such assembly is illustrated in FIG. 2 (described in more detail below). Although the assembly in FIG. 2 shows only one metal plate, other assemblies can have both front and back metal plates. A commercial imaging assembly of this type is KODAK EC-L.
Both portal imaging assemblies provide the desired high contrast images, but because of the thick metal screens used to provide the desired imaging features, they are very heavy and difficult and unsafe to carry throughout medical facilities. Medical users have tolerated this disadvantage because it has been believed that the thick metal plates are necessary for desired imaging properties.
We have found that this is not the case and it is to this problem that the present invention is directed.
The present invention provides an imaging assembly comprising a metal intensifying screen that is positioned adjacent to a fluorescent intensifying screen, the metal intensifying screen having a thickness of from about 0.1 to 0.75 mm when composed of copper or a thickness of from about 0.05 to 0.40 mm when composed of lead.
The present invention also provides a radiographic imaging assembly comprising a dual emulsion-coated radiographic silver halide film provided in combination with a metal intensifying screen that is positioned adjacent to the frontside of the radiographic film, and a fluorescent intensifying screen that is interposed between the radiographic silver halide film and the metal intensifying screen, the metal intensifying screen having a thickness of from about 0.1 to 0.75 mm when composed of copper or a thickness of from about 0.05 to 0.40 mm when composed of lead.
Still again, this invention provides a method of confirming the targeting of X-radiation comprising:
A) positioning the radiographic imaging assembly described above to a region of a subject,
B) directing the X-radiation at the region of the subject containing features that are identifiable by differing levels of X-radiation absorption and creating a first image of X-radiation penetrating the subject with the radiographic film as part of the radiographic imaging assembly described above,
C) directing X-radiation at the region of the subject and creating a second image superimposed on the first image in the radiographic film,
D) processing the radiographic film to obtain a viewable image from which intended targeting of the X-radiation directed at the region in relation to identifiable features of the subject is realized,
wherein during steps A and B, total X-radiation exposure is limited to 10 seconds or less.
The present invention provides a means for portal imaging using a light weight imaging assembly (or cassette) that has the desired imaging properties. It was a surprise to us that the thickness of at least the front metal intensifying screen could be significantly reduced from 1 mm without any significant loss in imaging properties or photographic speed. This invention will allow users to achieve portal imaging with lightweight portal imaging systems that are safer and easier to handle.